A brake system of this type is disclosed in German patent No. 34 39 408. The known brake system operates by the feedback principle during brake slip control operations. The return pump is of the self-priming type and aspirates pressure fluid from the low-pressure reservoir through a first suction valve and a first suction line for brake slip control. For traction slip control, the return pump aspirates pressure fluid from the pressure fluid reservoir through a second suction valve and a second suction line including a change-over valve, through the brake line and the master cylinder. Compared to other brake systems, in which the two suction lines are connected to conduct fluid through a common suction valve to the suction side of the pump and in which an additional suction valve is arranged in the first suction line preventing the development of a vacuum in the low-pressure accumulator, the parallel arrangement of the suction valves has the advantage that only one suction valve must be overcome during brake slip control and traction slip control operations. This reduces throttling effects, so that the delivery rate of the pump is improved.
A pump of this type is disclosed in German patent application, No. 40 27 794, for example. The pump described in the application is a radial piston pump with a delivery piston on which the valve seat for the suction valve is provided. The closure member of the suction valve is acted upon by a valve spring supported on the housing. The preload of the valve spring increases when the pressure chamber is decreased by a movement of the piston. The preload of the valve spring is at its maximum at the upper dead center of the piston, i.e. when the pressure chamber has its smallest volume. Thus, the opening pressure of the suction valve to be overcome is greatly increased compared to the lower dead center of the piston.
To provide a constant opening pressure of the suction valve, attempts have been made to captivate the valve spring of the suction valve. However, even this arrangement does not achieve the optimal flow characteristic curve of a pump of this type that is intended for use in a traction-slip controlled brake system, for example.
An object of the present invention is to provide a favorable structure of a generic brake system.
Another object of the present invention is to provide a hydraulic pump wherein the pilot pressure of the suction valve proximate the upper dead center of the delivery piston is lower than the pilot pressure proximate the lower dead center.
The integration of both suction valves in the pump housing permits an easy assembly because there is no need for additional valves in the suction lines.
In an arrangement which is especially favorable in terms of mounting space, one of the suction valves is provided on the movable pump piston and the other one is arranged on the housing.
It is an advantage that the suction valves are axially opposed because, as a result, the axial dimensioning of a cylindrical pressure chamber is regardless of the position of the suction valves.
The difference of the pilot pressures during a brake slip control operation and a traction slip control operation can be produced by differently dimensioning the opening cross-sections of the suction valves.
Thus, an application of both closure members of the suction valves by a common compression spring in the direction so as to close is possible, because the opening pressures are determined by the different opening cross-sections.
Even when using a common compression spring for both closure members, the load of the closure member of the second valve can be reduced by a second compression spring, which is weaker than the compression spring interposed between the closure members, counteracting the interposed compression spring.
The use of balls as closure members, instead of specially designed closure members, saves costs.
A variable captivation of the valve spring is principally achieved by a lever assembly on the pump. Proximate the upper dead center of the delivery piston, the valve spring is retained by the lever elements, thereby permitting the aspiration of fluid at a low pilot pressure already at the beginning of a piston movement away from the upper dead center. Thus, the total difference in pressure between the suction port and the pressure chamber contributes to accelerating the pressure fluid, without suffering from valve pilot pressures which must be overcome. Proximate the lower dead center, the valve spring is largely relieved from load, so that a further application by the lever elements is not required.
An application of the lever elements is disclosed and is particularly advantageous in a space-saving positioning of the valve spring within a compression spring that resets the delivery piston. In this arrangement, the variable force, which is responsive to the stroke of the delivery piston, can be generated by the compression spring that resets the delivery piston, or by a stop on the housing.
The present invention permits mounting the lever elements in one assembly operation. The integral manufacture of all lever elements and the interposed connecting portions eliminates the need for premounting the lever assembly.
It is particularly inexpensive to manufacture the one-piece transmitting element as a sheet-metal punched part.
An additional force is superimposed on the force exerted by the valve spring. The magnitude and direction of the additional force is variable as a function of the speed and/or the acceleration of the delivery piston, thereby facilitating the opening of the valve during the suction stroke.
This arrangement is achieved in a particularly simple and low-cost fashion by a friction element applying a specific friction resistance to the closure member. Thus, the closure member is exposed to a force directed opposite to the force of the valve spring during the suction stroke. The friction element can be positioned in the area of an element that guides the closure member, for example.
An additional force that is proportional to the speed of the delivery piston can be achieved by connecting the closure member with an element having a large flow resistance during the movement of the piston from the upper to the lower dead center. Advantageously, the design of the element is aerodynamically more favorable in the opposite direction. The aerodynamically unfavorable element can be designed integrally with the closure member.
According to another measure by which an additional force can be easily exerted on the closure member, the closure member is connected with an inertia member having a great mass compared to the mass of the closure member. Favorably, the inertia member can be arranged in the dead space produced by the overall length of the valve closure spring. Thus, additional structural volume is not required.
A preloading force which is constant over the entire suction stroke of the delivery piston and acts upon the closure member can be achieved by conforming the mass of the inertia member to the rated rotational speed of a motor driving the pump.
Accordingly, the force, caused by the inert mass, and the variable portion of the spring force, caused by the compression of the valve spring, are conformed to one another such as to neutralize, with the operational rotational speed predetermined. Thus, the delivery rate of the pump can be maximized with this predetermined operational rotational speed.
It is self-explanatory that the proposed individual measures can be combined advantageously.